.::VOLUME 16, LESSON 4::.
Mechanisms of Radiopharmaceutical Localization
Continuing Education for Nuclear Pharmacists
And
Nuclear Medicine Professionals
By
James A. Ponto, MS, RPh, BCNP
Chief Nuclear Pharmacists, University of Iowa Hospitals and Clinics and
Professor (Clinical), University of Iowa Hospitals and Clinics
The University of New Mexico Health Sciences Center, College of Pharmacy is accredited by the Accreditation
Council for Pharmacy Education as a provider of continuing pharmacy education. Program No. 0039-000-12-164H04-P 2.5 Contact Hours or .25 CEUs. Initial release date: 7/19/2012
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Mechanisms of Radiopharmaceutical Localization
By
James A. Ponto, MS, RPh, BCNP
Editor, CENP
Jeffrey Norenberg, MS, PharmD, BCNP, FASHP, FAPhA
UNM College of Pharmacy
Editorial Board
Stephen Dragotakes, RPh, BCNP, FAPhA
Michael Mosley, RPh, BCNP
Neil Petry, RPh, MS, BCNP, FAPhA
James Ponto, MS, RPh, BCNP, FAPhA
Tim Quinton, PharmD, BCNP, FAPhA
S. Duann Vanderslice, RPh, BCNP, FAPhA
John Yuen, PharmD, BCNP
Advisory Board
Dave Engstrom, PharmD, BCNP
Vivian Loveless, PharmD, BCNP, FAPhA
Brigette Nelson, MS, PharmD, BCNP
Brantley Strickland, BCNP
Susan Lardner, BCNP
Christine Brown, BCNP
Director, CENP
Kristina Wittstrom, MS, RPh, BCNP, FAPhA
UNM College of Pharmacy
Administrator, CE & Web Publisher
Christina Muñoz, M.A.
UNM College of Pharmacy
While the advice and information in this publication are believed to be true and accurate at the time of press, the author(s), editors, or the
publisher cannot accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty,
expressed or implied, with respect to the material contained herein.
Copyright 2012
University of New Mexico Health Sciences Center
Pharmacy Continuing Education
MECHANISMS OF RADIOPHARMACEUTICAL LOCALIZATION
STATEMENT OF LEARNING OBJECTIVES:
Upon successful completion of this lesson, the reader should be able to:
1.
Describe common mechanisms of biologic localization.
2. Describe, for common radiopharmaceuticals, its mechanism of localization and expected
biodistribution.
3. Describe examples of altered radiopharmaceutical biodistribution related to improper product
preparation.
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COURSE OUTLINE
INTRODUCTION .................................................................................................................................................................. 7 COMPARTMENTALIZED .................................................................................................................................................. 7 UNIFORM DISPERSION WITHIN A COMPARTMENT ................................................................................................................ 8 NON-UNIFORMITIES WITHIN THE COMPARTMENT ............................................................................................................... 8 LEAKAGE FROM THE COMPARTMENT................................................................................................................................. 11 MOVEMENT (FLOW) WITHIN A COMPARTMENT ................................................................................................................. 12 PASSIVE DIFFUSION ........................................................................................................................................................ 15 FILTRATION....................................................................................................................................................................... 18 FACILITATED DIFFUSION ............................................................................................................................................. 19 ACTIVE TRANSPORT ....................................................................................................................................................... 19 SECRETION ........................................................................................................................................................................ 21 PHAGOCYTOSIS ................................................................................................................................................................ 22 CELL SEQUESTRATION .................................................................................................................................................. 23 CAPILLARY BLOCKADE ................................................................................................................................................ 24 ION EXCHANGE ................................................................................................................................................................ 25 CHEMISORPTION ............................................................................................................................................................. 25 CELLULAR MIGRATION ................................................................................................................................................ 26 RECEPTOR BINDING ....................................................................................................................................................... 26 ALTERED BIODISTRIBUTION RELATED TO IMPROPER PREPARATION ....................................................... 28 INTERESTING CASE......................................................................................................................................................... 30 SUMMARY .......................................................................................................................................................................... 30 REFERENCES ..................................................................................................................................................................... 31 ASSESSMENT QUESTIONS ............................................................................................................................................. 32 -Page 5 of 35-
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INTRODUCTION
Nuclear pharmacists must understand how radiopharmaceuticals work; i.e., their mechanism of action
(or more appropriately, their mechanism of localization). Such knowledge is necessary to understand
the performance of clinical nuclear medicine procedures, many drug-radiopharmaceutical interactions,
and other causes of altered biodistribution.
The following descriptions and examples are of the author’s own categorization; they are similar to,
but not the same as, those found in textbooks and review articles1-4. These mechanisms are not unique
for radiopharmaceuticals, but may be applicable for the description of localization mechanisms for
other substances including conventional drugs.
It should be emphasized that the localization of most radiopharmaceuticals is not limited to one simple
mechanism, but also involves other processes such as delivery to the tissue and retention in the cells.
Moreover, the localization of some radiopharmaceuticals may involve the combination of more than
one mechanism.
In each of the following sections, the mechanism is described, its characteristics are detailed, and
examples are presented. Although the majority of radiopharmaceuticals are mentioned, not all
radiopharmaceuticals or all uses of certain radiopharmaceuticals are included. Rather, the intent is to
provide a foundation and framework for understanding mechanisms of localization of existing and
most future radiopharmaceuticals.
COMPARTMENTALIZED
Compartmentalized, or compartment localization, refers to the situation where the molecules of interest
are distributed in an enclosed volume. The space in which the volume is enclosed is the compartment.
Examples of common biologic compartments are listed in Table 1.
Table 1
Examples of common biologic compartments
blood pool (vasculature)
lung airways
cerebrospinal fluid (CSF) space
peritoneal cavity
Gastrointestinal tract
urinary tract
lymphatic channels
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Uniform Dispersion Within a Compartment
The classic example of uniform dispersion within a compartment involves the blood pool. The
quantitative determination of blood volume can be performed by employing the tracer dilution method.
I-125 radioiodinated serum albumin (I-125 RISA), a radiopharmaceutical which disperses in the
plasma, is used to determine plasma volume. Cr-51 labeled red blood cells, a radiopharmaceutical that
disperses within the cellular content of blood, is used to determine red cell volume (sometimes referred
to as red cell mass).
Tc-99m red blood cells (RBCs) are dispersed in the blood and used in gated blood pool imaging of left
ventricular wall motion and determination of left ventricular ejection fraction. Figure 1 shows Tc99m RBCs in the left ventricular blood pool at end-diastole (when the heart relaxed and the chamber is
full of blood) and at end-systole (when the heart has contracted and only residual blood remains in the
chamber), along with a graphical representation of activity vs. time from which ejection fraction is
calculated.
Figure 1. Tc-99m RBCs showing left ventricular blood pool (arrows) at end-diastole (A) and at end-systole (B).
Also shown is activity/time graph from which ejection fraction (EF) is calculated (C).
Non-Uniformities Within the Compartment
In some situations, radiopharmaceuticals may demonstrate non-uniformities within the compartment
thus reflecting pathophysiology. Areas of increased radiopharmaceutical concentration may reflect
pathologic changes in the tissue or organ. For example, a localized area of increased Tc-99m RBCs
activity can be caused by increased blood volume in a hemangioma (see Figure 2).
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Another example involves cases of hydronephrosis, which causes dilatation of the collecting system in
the urinary tract. In this situation, there will be increased urine volume, and thus increased
radioactivity concentration of Tc-99m pentetate (DTPA) or Tc-99m mertiatide (MAG3). Such
localized increased radioactivity in the collecting system or ureter could also be caused by an ureteral
obstruction. Non-obstructive hydronephrosis can be differentiated from ureteral obstruction by its
washout in response to furosemide (see Figure 3).
Figure 2. Increased
concentration of Tc-99m RBCs in
a hemangioma near the right
knee (arrows).
Figure 3. (A) Increased concentration of Tc-99m DTPA in
the right renal pelvis and left proximal ureter (arrows). (B)
Washout of activity down to the bladder in response to
furosemide confirms non-obstructive hydronephrosis.
In other situations, pathophysiology may be demonstrated as areas of decreased radiopharmaceutical
concentration. Such areas of decreased radiopharmaceutical concentration are most commonly the
result of an obstruction in a compartmental space. One example is obstructions in the lung airways
demonstrated by Xe-133 ventilation lung imaging. If complete obstruction, there will be absence of
Xe-133 in the area beyond the site of airway obstruction. If partial obstruction (eg, frequently seen in
chronic obstructive pulmonary disease [COPD]), there will be absence of Xe-133 in the affected area
upon initial inhalation and breath-hold, but Xe-133 gas will pass through the site(s) of partial
obstruction over time during equilibrium rebreathing (see Figure 4).
Obstructions can also occur in the CSF space. Following intrathecal injection, In-111 pentetate
(DTPA) flows up the spine and throughout the brain. In the case of obstructive hydrocephalus, the
obstruction prevents migration of In-111 DTPA to the upper part of the brain (see Figure 5).
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Figure 4. Xe-133 ventilation lung imaging shows areas
of airway obstruction during inhalation breath-hold
(arrows) (A) which normalize during equilibrium
rebreathing (B), thus indicating that these are sites of
partial obstruction.
Figure 5. (A) Normal migration of In-111 DTPA
throughout the CSF space in the brain. (B) Absent
migration of In-111 DTPA in the upper part of the brain
(arrows) in a patient with obstructive hydrocephalus.
The biliary tract is another compartment in which an obstruction may be present. Tc-99m
hepatobiliary radiopharmaceuticals, disofenin (DISIDA) and mebrofenin (BRIDA), are excreted from
the liver into the bile and flow through the biliary tract with normal flow into the gallbladder and into
the intestine. If the cystic duct is obstructed, there will be lack of radiopharmaceutical in the
gallbladder, and if the common bile duct is obstructed, there will be lack of radiopharmaceutical in the
small intestine (see Figure 6).
One final example of an obstruction in a compartment is that of the urinary tract. An obstruction of a
ureter will prevent transit of urine to the bladder. Such ureteral obstructions can be seen with renallyeliminated radiopharmaceuticals (see Figure 7). In addition to the obvious radiopharmaceuticals
excreted in the urine, Tc-99m DTPA and MAG3, several others also exhibit substantial elimination in
the urine including Tc-99m bone agents (medronate [MDP]) and oxidronate [HDP]) and F-18
fludeoxyglucose (FDG).
Figure 6. (A) Nonvisualization of gallbladder (arrow) because of cystic
duct obstruction. (B) Nonvisualization of activity in the small intestine
because of common bile duct obstruction (arrow).
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Figure 7. Tc-99m MDP
bone scan showing
previously unsuspected
ureteral obstruction (arrow).
Leakage From the Compartment
In some pathologic conditions there is abnormal leakage of contents from the compartment.
Radiopharmaceuticals offer a good method to detect and identify the location of such leakage. For
example, in cases of gastrointestinal hemorrhage (GI bleeding), blood extravasates from the
vasculature and accumulates in the GI tract. Tc-99m RBCs can be used to visualize the site(s) of the
GI bleed (see Figure 8).
Another example of leakage from a compartment in pathologic conditions is cerebrospinal fluid (CSF).
Imaging of In-111 DTPA following intrathecal injection may demonstrate CSF leakage (see Figure 9).
One potential complication of a cholecystectomy or other abdominal surgery is an iatrogenic bile leak.
Tc-99m disofenin or mebrofenin can used to visualize whether the bile remains in the biliary tract or if
leaks out into the abdominal cavity (see Figure 10).
Figure 8. (A) Normal distribution of Tc-99m
RBCs in the blood pool. (B) Accumulation of
Tc-99ms RBCs at two sites of GI bleeding
(arrows).
Figure 9. Leakage of In-111
DTPA into nasal pharynx with
accumulation in
oropharynx/mouth (arrows).
Figure 10. Hepatobiliary
imaging shows bile leak into
the abdomen (arrows)
following cholecystectomy.
Similarly, a urine leak into the abdomen can result from a surgical complication, especially related to
surgeries involving the kidneys and urinary tract. Such urine leaks may be visualized with Tc-99m
DTPA or MAG3, although MAG3 is preferred because it has much higher concentration in the urine
(see Figure 11).
As one more example of leakage from a compartment, peritoneal fluid may leak through a
communication across the diaphragm and accumulate in the pleural cavity causing hydrothorax. Such
peritoneal-plural communication can be demonstrated by imaging Tc-99m sulfur colloid following
intraperitoneal injection (see Figure 12).
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Figure 11. Tc-99m MAG3 SPECT
coronal slice of kidney shows leakage
of radioactive urine MAG into the
abdominal cavity (arrows).
Figure 12. Following
intraperitoneal injection of Tc-99m
sulfur colloid, activity is seen in the
right pleural cavity (arrow).
Movement (Flow) Within a Compartment
In some situations, pathologic conditions may be evaluated by assessing alterations in the direction,
rate, and extent of flow within a compartment. For example the rate of emptying of gastric contents
into the intestine can be determined by imaging radiopharmaceuticals in the stomach. Tc-99m sulfur
colloid is the preferred radiopharmaceutical because it is non-absorbable from the GI tract. Tc-99m
sulfur colloid bound in scrambled eggs can be used to evaluate gastric empting of food solids while
Tc-99m sulfur colloid mixed in water (or other liquid such as juice or baby formula) can be used to
evaluate gastric emptying of liquids (see Figure 13 and Figure 14). Individual patient gastric emptying
is compared to normal values, which are typically taken to be +/- 2 standard deviations of the mean of
normal controls.
Figure 13. Tc-99m sulfur colloid administrated orally in water shows marked
abnormally slow rate of gastric emptying (almost no emptying during 1 hour).
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Figure 14. Tc-99m sulfur colloid administrated orally in water shows marked
abnormally fast rate of gastric emptying (essentially complete emptying
within 15 minutes).
The contractile function of the gallbladder may be impaired in conditions such as chronic cholecystitis.
Following injection of a hepatobiliary radiopharmaceutical and its accumulation in the gallbladder, the
contractile function of the gallbladder can be assessed by its response to sincalide infusion. Sincalide,
the synthetic octapeptide of cholecystokinin, stimulates gallbladder contraction and emptying. The
extent of emptying, typically in terms of the emptying fraction (EF), can be determined by inspection
of a radioactivity vs. time curve (see Figure 15).
Many malignant tumors tend to spread through lymphatic channels to regional lymph nodes. Hence,
delineation of lymphatic drainage patterns may be important for selecting biopsy sites. Small
particulate radiopharmaceuticals are suitable for visualizing lymphatic drainage from tumor sites.
Because lymphatic channels are about 200 microns in size, Tc-99m sulfur colloid filtrate from passage
through a 0.1 micron or a 0.2 micron filter is widely used. Following intradermal, subcutaneous, or
interstitial injections of filtered Tc-99m sulfur colloid around or near the tumor site, lymphatic
drainage channels to regional lymph node beds can be observed (see Figure 16).
Ureteral reflux refers to the abnormal condition of urinary bladder contents refluxing back up the
ureter(s) to the kidney(s). Such reflux may contribute to recurrent kidney infections by offering
transport to bacteria harbored in the bladder. The presence and extent of reflux can be evaluated by
using a radiopharmaceutical instilled via a urinary catheter into the bladder (see Figure 17). Some
centers use Tc-99m sodium pertechnetate while others prefer Tc-99m sulfur colloid.
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Figure 15. Imaging of Tc-99m mebrofenin in this normal individual shows
essentially complete gallbladder emptying in response to sincalide.
Figure 16. Following injection of 0.1-micron filtered Tc-99m sulfur colloid around
an abdominal melanoma (arrow), flow is seen in one lymphatic drainage channel
going to the right groin and in two lymphatic drainage channels both going to the
left axilla (arrow heads).
The eyes are coated by tears secreted by the lacrimal glands. Excess tears drain from the eye through
the nasolacrimal drainage duct to the nasopharynx. Obstruction of the drainage duct can produce
epiphora (tears running out of the eyes and down the cheeks). Epiphora can also result from excess
tear production (eg, crying). To differentiate, a drop of radiopharmaceutical can be placed on the eye
and images obtained to see whether or not the drainage duct is patent (see Figure 18). Some centers
use Tc-99m pertechnetate while others prefer Tc-99m sulfur colloid.
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Evaluation of the patency of artificial shunts is one more example of assessing flow within a
compartment. One treatment for ascites (abnormal fluid accumulation in the peritoneal cavity) is the
placement of a catheter (shunt) from the peritoneal cavity to the subclavian vein. Excess ascitic fluid
can then drain through the peritoneal-venous shunt into the blood where it can be eliminated in the
urine. Evaluation of these shunts can be performed following intraperitoneal injection of Tc-99m
macroaggregated albumin (MAA). If the shunt is patent, MAA will flow through it into the venous
system, through the right side of the heart, and into the lungs where it lodges (see Figure 19).
Figure 17. Following
installation of Tc-99m
sulfur colloid into the
bladder, reflux in seen in
both ureters ascending to
the renal pelvis on both
sides (arrows).
Figure 18. Following a
drop of Tc-99m sulfur
colloid placed onto the
surface of each eye, images
show transit of radioactivity
through the drainage duct of
the left eye but absence of
drainage from the right eye
due to obstruction of the
right drainage duct (arrows)
Figure 19. (A) Following intraperitoneal injection of
Tc-99m MAA, activity in seen in the shunt tubing
(arrow) and in the lungs, which indicates normal
patency. (B) Following intraperitoneal injection of
Tc-99m MAA, activity is seen in the shunt tubing
until a site of obstruction (arrow) but no further.
PASSIVE DIFFUSION
Passive diffusion can be described as the random movement of molecules with the net effect toward
achieving uniform concentration. A well-known example of passive diffusion is the movement of tea
from a tea bag into and throughout a container of water.
In the context of biological systems, passive diffusion typically involves passage across a membrane.
Several factors are involved in the ability of molecules to cross membranes. First is lipid solubility.
Because membranes are composed primarily of phospholipids, molecules that are highly lipid soluble
can usually ‘dissolve’ through and across membranes whereas hydrophilic polar molecules cannot. A
second factor is pH/ionization. Many molecules may exist in either a neutral state or in a charged ionic
state, depending on pH. For example, an amine can be neutral at a higher pH, but becomes protonated
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at a lower pH. Hence, depending on the pH of the immediate environment, a molecule may be able to
diffuse across a membrane in its unionized, lipophilic form but cannot diffuse across the same
membrane in its ionized, hydrophilic form. A third factor is molecular size. Many membranes have
small pores, or holes, that allow certain small molecules to pass through. However, this is generally
limited to molecules having a molecular weight of less than 80 daltons.
Passive diffusion can be further described in terms of several characteristics. First, passive diffusion
requires a concentration gradient; that is, the net flow of molecules is from an area of high
concentration to an area of low concentration. In biological systems, membranes typically separate
these areas of high concentration vs. low concentration, so diffusion occurs across the membrane.
Second, the rate of diffusion is a function of the concentration gradient; that is, the rate is faster at
higher concentration gradients and is slower at lower concentration gradients. Third, it is a passive
process involving only molecular motion and does not require the input of other external energy.
Fourth, no transporters, carriers, or other receptors are involved so passive diffusion is non-selective, is
not competitively inhibited by similar molecules, and is not subject to saturation.
A classic example of passive diffusion in nuclear medicine is Tc-99m DTPA brain imaging. Tc-99m
DTPA cannot normally penetrate the blood-brain barrier (BBB), so it normally remains in the blood
pool until cleared by the kidneys. In conditions that result in disruption of the BBB, such as tumor,
stroke, and infection, the Tc-99m DTPA can diffuse across the disrupted BBB and accumulate in that
affected area of the brain (see Figure 20).
Figure 20. (A) Following IV injection of Tc-99m DTPA,
dynamic imaging shows poor cerebral blood flow to an area
of stroke (arrows). (B) Delayed imaging at 3 hours after
injection shows increased accumulation of Tc-99m DTPA in
the same area of stroke (arrow) because of diffusion across
a disrupted BBB.
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In many cases, passive diffusion necessarily
requires delivery of the molecule to the
location of interest. Moreover, because
diffusion is not a unidirectional process,
accumulation typically involves some method
Figure 21. (A) Normal uptake of Tc-99m HMPAO in the
brain of a living person. (B) Absence of Tc-99m HMPAO
uptake in the brain is consistent with brain death. Also
seen is normal uptake in lacrimal glands, nasopharynx,
thyroid, and lungs.
of retention. Hence, passive diffusion is often
inter-related with delivery (eg, blood flow to
the area) and retention of the material in the tissue of interest. For example, localization of the cerebral
perfusion radiopharmaceuticals Tc-99m exametazime (HMPAO) and Tc-99m bicisate (ECD) involves
delivery via cerebral arterial blood flow, diffusion into the brain, and retention in the brain due to
conversion to a hydrophilic species and enzymatic metabolism, respectively. Whole brain cerebral
perfusion can be assessed for evaluation of brain death (see Figure 21), as well as evaluation of altered
cerebral perfusion to discrete areas within the brain (see Figure 22).
Figure 22. SPECT imaging following ictal injection of Tc-99m
ECD shows increased uptake in the seizure focus (arrow).
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Figure 23. Following injection of Tc-99m
sestamibi, activity is seen in the heart,
salivary glands, thyroid, skeletal muscle (note
lack of uptake in bone), excretion by the
kidneys into the urine with collection in the
bladder, excretion by the liver into the bile
with collection in the gallbladder and
intestines, and infiltration at the injection site
Tc-99m myocardial perfusion agents are another group of radiopharmaceuticals that depend on
delivery (ie, blood flow through the coronary arteries), diffusion into myocardial cells, and retention in
those cells. Both Tc-99m sestamibi and Tc-99m tetrofosmin cross the cell membranes by lipophilic
diffusion and then are retained by electrostatic binding to negative electrical charges on the
mitochondrial membranes. Because only about 5% of cardiac output goes into the coronary arteries,
the majority of these radiopharmaceuticals goes elsewhere in the body (see Figure 23).
FILTRATION
Filtration refers to a special case of diffusion involving transit of molecules through pores, or channels,
driven by a hydrostatic or osmotic pressure gradient. This prime example of this is glomerular
filtration by the kidney. There are two primary factors involved in filtration. The first is molecular
size vs. pore size. For glomerular filtration, only small hydrophilic molecules (i.e., molecular weight
of <5000) are typically able to pass through the glomerular pores. The second factor is availability.
For glomerular filtration, only those molecules that are free in plasma (i.e., not protein bound) are
available to be filtered.
Filtration can be further described in terms of several
characteristics. First, filtration requires some sort of force or
pressure gradient. For glomerular filtration, this is blood
pressure. However, it does not require the local input of
other external energy. Third, no transporters, carriers, or
other receptors are involved so filtration is non-selective, is
not competitively inhibited by similar molecules, and is not
subject to saturation.
Although many radiopharmaceuticals are excreted, at least in
part, by glomerular filtration, the radiopharmaceutical used
for glomerular function renal imaging is Tc-99m DTPA (see
Figure 24).
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Figure 24. Following pre-treatment with
captopril, decreased glomerular filtration
of Tc-99m DTPA is seen in the left
kidney (arrow). As an ACE inhibitor,
captopril blocked the compensatory
mechanism activated by left renal artery
stenosis, resulting in decreased
glomerular pressure in the left kidney.
FACILITATED DIFFUSION
Facilitated diffusion is a type of carrier-mediated transport across
membranes. It can be thought of as a revolving door. There are several
characteristics that help define facilitated diffusion. Importantly, a carrier
in the membrane is used to carry the molecule across the membrane.
Because a ca rrier is used, it is selective (i.e., only certain molecules fit
into the carrier). Accordingly, it can be competitively inhibited by the
presence of similar molecules that also fit into the carrier. Because there
are a limited number of carriers, it is possible to achieve saturation (i.e.,
the maximum response when all carriers are engaged). Facilitated
diffusion uses carriers that are passive (i.e., like an un-motorized
revolving door), so it requires a concentration gradient to operate.
However, it does not use external energy.
The prime example of facilitated diffusion is glucose. Glucose enters
cells via transmembrane protein transporters [GLUT]. The radiolabeled
analog of glucose, F-18 fludeoxyglucose (FDG), similarly enters cells via
the glucose transporters [GLUT]. Once inside cells, both glucose and
FDG are phosphorylated by hexokinase. Glucose-6-phosphate then
enters the glycolytic pathway. Further metabolism of FDG-6-phosphate
Figure 25. Following injection
of F-18 FDG in a normal
patient, there is high uptake in
brain, variable uptake in heart
(high uptake in this patient),
and moderate uptake in liver,
GI tract, and marrow. F-18
FDG filtered by the kidneys is
not reabsorbed by the distal
tubules so it remains in the
urine, demonstrating activity in
the renal collecting system,
ureters, and bladder.
is blocked, however, so it is retained in the cells. Hence, cellular uptake
reflects glucose metabolism (see Figure 25). It is important to remember that glucose and FDG are
competing for the same GLUT transporters, so elevated blood levels of glucose will decrease cellular
uptake of FDG.
ACTIVE TRANSPORT
Active transport is another type of carrier-mediated transport across membranes. Unlike facilitated
diffusion, however, it requires energy, usually from ATP, for the transporters to function. Hence, it
can be thought of as a motorized revolving door or as a pump. Using energy-dependent carriers allows
the transport of molecules against a concentration gradient (i.e., from an area of low concentration to
an area of high concentration). Other characteristics of active transport, however, are similar to
facilitated diffusion. Because a carrier is used, it is selective (i.e., only certain molecules fit into the
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carrier). Accordingly, it can be competitively inhibited by the presence of similar molecules that also
fit into the carrier. Because there are a limited number of carriers, it is possible to achieve saturation
(i.e., the maximum response when all carriers are engaged).
A well-known example of active transport is the concentration of iodide in the thyroid gland. Iodide
ions are transported into thyroid cells via the Na+/I- symporter. Hence, radioisotopes of iodine such as
I-123 and I-131 are useful radiopharmaceuticals to
evaluate thyroid function. Additionally, Tc-99m
pertechnetate (TcO4-) is of similar ionic radius and
a negative charge so it is also transported like
iodide (see Figure 26). It should be emphasized
that high concentrations of iodide in the blood,
such as from injection of iodinated x-ray contrast
media, will competitively inhibit thyroid uptake of
these radiopharmaceuticals (see Figure 26).
Figure 26. (A) Normal uptake of Tc-99m pertechnetate
in thyroid (and salivary glands). (B) Absent thyroid
uptake (arrow) of Tc-99m pertechnetate in a patient
who was administered iodinated x-ray contrast media a
few days before.
A second example of active transport is glucose absorption from the GI tract into the blood and
reabsorption of glomerular-filtered glucose back into the blood by the distal renal tubules. This is
accomplished using the sodium-dependent glucose cotransporter (SGLT). Although F-18 FDG is a
ligand for facilitated diffusion via GLUT transporters, it is not readily transported by SGLTs.
Therefore, glomerular filtered F-18 FDG remains in the urinary tract and flows to the bladder rather
than being reabsorbed into the blood as is glucose (see Figure 25).
Another well-known example of active transport is
the Na+/K+ pump, especially of importance in the
heart muscle. Thallous chloride Tl-201 has been
widely used for myocardial perfusion scans.
Thallium is not a chemical analog of potassium,
but rather is a transition metal. However, thallous
ion (Tl+) happens to have an ionic radius similar to
Figure 27. (A) Reduced uptake of Tl-201 in an area
of heart muscle (arrow) due to stress-induced
ischemia in a patient with coronary artery disease.
(B) Normalization of uptake following redistribution
in the same patient.
K+ and fits in the Na+/K+ pump. Uptake in heart
muscle demonstrates viability. Also important is
delivery to the myocardial cells via blood flow
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through the coronary arteries. Hence, uptake in heart muscle also reflects coronary perfusion (see
Figure 27). A second radiopharmaceutical is rubidium chloride Rb-82, which is used for PET
myocardial perfusion scans. Rubidium is a chemical analog of potassium, falling immediately below it
on the periodic table of the elements, and Rb+ also fits in the Na+/K+ pump.
A fourth example of active transport is the norepinephrine transporter in adrenergic nerve terminals.
This transmembrane carrier transports monoamine neurotransmitters into neurons where they are
accumulated in storage vesicles. I-123metaiodobenzylguanidine (MIBG) is structurally related to
norepinephrine and fits in this transporter. Over-expression of this transporter occurs in certain
neoplasms such as neuroblastoma and pheochromocytoma (see Figure 28).
A final example of active transport involves the tubular cells in the renal cortex. Tc-99m succimer
(DMSA) is taken up by, and retained in, these cells (see Figure 29).
Figure 28. I-123 MIBG
demonstrates increased uptake
in a pheochromocytoma in the
right adrenal gland (arrow).
Normal uptake is also seen in
salivary glands, liver, heart, and
bowel.
Figure 29. Tc-99m DMSA renal scans
shows normal uptake throughout the renal
cortex. Areas of decreased uptake in
both the upper and lower poles of the
right kidney (arrows) are scar formation
subsequent to recurrent kidney infections.
SECRETION
Secretion refers to the special case of active transport out of glands and other tissues. Three common
examples are secretion of hydrochloric acid by the stomach, secretion of certain substances by the
kidney tubular cells into the urine, and secretion of certain substances by the liver in the bile.
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Meckel’s Diverticulum is a patch of ectopic stomach tissue, usually in the intestine. Being stomach
tissue, it secretes hydrochloric acid which, being in the intestine, erodes the intestinal wall to the point
of bleeding. Tc-99m pertechnetate (TcO4-) is of similar size and like charge to chloride (Cl-) so it is
secreted as pertechnic acid (H+ TcO4-) by both normal stomach tissue and Meckel’s Diverticuli (see
Figure 30).
The tubular cells in the kidneys secrete certain substances, such as some waste products, into the
urinary collecting system. Tc-99m mertiatide (MAG3) is cleared from the blood by this mechanism,
resulting in much higher urinary concentrations and better contrast compared to radiopharmaceuticals
eliminated by glomerular filtration (see Figure 31).
The hepatocytes in the liver extract certain substances from the blood and secrete them into the bile.
Tc-99m IDA agents (disofenin, mebrofenin) are excreted using the same organic ion transport system
as is used by bilirubin (see Figure 32).
Figure 30. Tc-99m sodium
pertechnetate scan in child
with GI bleeding shows
marked uptake in a Meckel’s
Diverticulum (arrow). Also
seen is normal retention in
blood pool, uptake in
stomach, and excretion in
urine (bladder).
Figure 31. Normal Tc-99m MAG3 renal
scan showing prompt extraction from the
blood and secretion into the urine.
Figure 32. Hepatic uptake of Tc99m mebrofenin in a transplanted
liver indicates organ viability
(because active
transport/secretion requires
energy, the tissue is alive).
PHAGOCYTOSIS
The term phagocytosis comes from Greek words that translate as ‘cell eating.’ It is the process
whereby the cell engulfs a particle and internalizes it. A prime example involves reticuloendothelial
system (RES) cells, such as Kupffer cells in the liver, phagocytizing colloid particles. Although this
mechanism is potentially saturable, it is unlikely for the small doses of particles used in
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radiopharmaceuticals. Tc-99m sulfur colloid, the traditional radiopharmaceutical used for liver scans,
is localized by this mechanism. Focal areas lacking Kupffer cells, such a tumor, cyst, abscess, or
hemangioma, will be demonstrated as areas of lack of uptake (see Figure 33). If the entire liver is
poorly function ing, such as with hepatitis or cirrhosis, there is typically a ‘colloid shift’ – i.e., a shift in
normal uptake away from the liver with increased uptake in the spleen and bone marrow (see Figure
34).
Figure 33. Tc-99m sulfur colloid scan shows
normal uptake in liver but lack of uptake in a
cavernous hemangioma.
Figure 34. Tc-99m sulfur
colloid scan in a patient with
severe alcoholic cirrhosis
demonstrating colloid shift
away from the liver (arrow) to
the spleen and marrow.
CELL SEQUESTRATION
Cell sequestration refers to the process whereby old
or damaged red blood cells are removed from the
circulation. This process is generally associated
specifically with the spleen. Although this
mechanism is potentially saturable, it is unlikely for
the relatively small numbers of cells used for
Figure 35. Imaging with heat-damaged Tc-99m
RBCs in a post-splenectomy patient shows uptake
in an accessory spleen (arrows). Also seen is
normal retention in blood pool (heart, liver, and
major blood vessels).
imaging. The radiopharmaceutical is generally
prepared by labeling RBCs in vitro with Tc-99m
(using the modified Brookhaven labeling method
facilitated with use of commercial product
UltraTag™ and then damaging them by heating at 49oC for 15 minutes. This imaging procedure is
especially useful for localizing and/or identifying ectopic accessory spleens (see Figure 35).
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CAPILLARY BLOCKADE
Capillary blockade refers to the microembolization (i.e., physical trapping) of particles in capillaries
and pre-capillary arterioles. Because the diameter of capillaries and pre-capillary arterioles are about
10 microns and 20-30 microns, respectively, such particles should be slighter larger in size, for
example 10-50 microns. Obviously, delivery to the capillary bed via blood flow is also a necessary
component of this localization.
When such sized particles are injected intravenously, the first capillary beds encountered are the lungs.
Hence, radiolabeled aggregates (e.g., Tc-99m macroaggregated albumin [MAA]) have long been used
for perfusion lung imaging (see Figure 36).
Because this mechanism necessarily involves delivery to the capillary beds via blood flow, localization
of Tc-99m MAA in each of the two lungs is a surrogate for relative blood flow to each of the two
lungs. Hence, Tc-99m MAA perfusion lung imaging can also be used to assess blood flow through the
pulmonary arteries (see Figure 37).
A similar procedure can be used to evaluate blood flow within the liver. When Tc-99m MAA is
injected through a catheter positioned in the hepatic artery, it is delivered via hepatic blood flow to the
capillaries in the liver (see Figure 38).
Figure 36. Tc-99m MAA
perfusion lung scan
showing a classic wedgeshaped defect (arrow) in a
patient with pulmonary
embolism.
Figure 37. Tc-99m MAA
perfusion lung imaging in a
child with congenital heart
disease including left
pulmonary artery stenosis
shows differential perfusion to
be 23% left lung and 77% right
lung.
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Figure 38. Imaging of Tc-99m MAA injected into
hepatic artery shows the majority of uptake in the
left lobe of the liver (arrow), the area of known
metastases, with no significant shunting to the
lungs or GI tract. Because of this favorable
biodistribution, this patient was treated with Y-90
microspheres.
ION EXCHANGE
Ion exchange refers to the exchange of ionic chemical analogs, especially in a crystalline matrix.
Classic examples include exchange of Sr+2 for Ca+2 and F- for OH- in hydroxyapatite. Although
potentially saturable, it is unlikely for the small doses used in radiopharmaceuticals.
Current radiopharmaceuticals that localize in bone by this mechanism are Sr-89 strontium chloride, a
beta-emitter used to treat painful bone metastases, and F-18 sodium fluoride, a PET agent used for
bone scans.
CHEMISORPTION
Chemisorption refers to the binding of phosphate-type
compounds onto the surface of bone. The strength of this
binding is intermediate between chemical covalent bonding and
hydrogen bonding (adsorption), hence the coined term
chemisorption. Examples of such compounds are diphosponates
such as Tc-99m medronate (MDP) and oxidronate (HDP) for
bone imaging and Sm-153 lexidronam (EDTMP) for treatment of
painful bone metastases. Because localization is on the surface,
uptake reflects surface area. Hence, there is increased uptake in
areas of greater surface area associated with increased bone
metabolism (e.g., fracture, infection, tumor) (see Figure 39).
Figure 39. Tc-99m MDP bone scan
shows increased uptake in reactive
bone around metastatic tumors
(arrows). Note: there is lack of
primary uptake in the tumors
themselves. Also seen is normal
excretion into the urine (kidneys,
Although potentially saturable, it is unlikely for the small doses
used in radiopharmaceuticals.
In addition to chemisorption on the surface of bone, there can
also be chemisorption onto calcium phosphate crystals that
precipitate in certain tissues as a consequence of severe
hyperparathyroidism and hypercalcemia (see Figure 40).
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Figure 40. Tc-99m MDP scan in a
patient with hyperparathyroidism and
hypercalcemia shows uptake in sites
of metastatic calcification (i.e, calcium
phosphate precipitation), specifically
the lungs, stomach, and knee joints
(arrows).
CELLULAR MIGRATION
Cellular migration refers to the directed
migration of cells, especially in response to
stimuli. The prime example is the
chemotaxis of white blood cells to sites of
infection. Hence, autologous leukocytes
radiolabeled with In-111 oxine or Tc-99m
Figure 41. In-111 leukocyte scan in a patient with Crohn’s
disease shows uptake in a right abdominal abscess and in a
superficial infection at the colostomy site on the left side
(arrows). Also seen is normal uptake in spleen, liver, and
bone marrow.
exametazime (HMPAO) can be used to
localize sites of infection (see Figure 41).
RECEPTOR BINDING
Receptor binding refers to the “lock-and-key” binding of a
molecule to a specific receptor site. A classic example is the
binding interaction of an antibody (or antibody fragment) to an
antigen. Other examples of ligands that bind to particular
receptor sites include peptides/hormones and neurotransmitter
analogs. General characteristics of receptor binding include
selectivity, competitive inhibition by similar molecules, and the
possibility of being saturated.
Although much research has been conducted with radiolabeled
antibodies, few are currently marketed. In-111 capromab
pendetide, a monoclonal murine IgG antibody directed to
prostate specific membrane antigen (PSMA), is used for staging
Figure 42. In-111 capromab pendetide
image shows uptake in a prostate
cancer metastasis in the right lung
(arrow). Also seen is normal retention
in blood pool and normal uptake in liver,
spleen, and marrow.
and follow-up of prostate cancer (see Figure 42). I-131 tositumomab and In-111/Y-90 ibritumomab
tiuxetan, monoclonal murine IgG antibodies directed to CD20 receptors on B-cells and non-Hodgkin’s
lymphoma tumor cells, are used for treatment of non-Hodgkin’s lymphoma (see Figure 43).
Many neuroendocrine tumors exhibit enhanced expression of somatostatin receptors. In-111
pentetreotide, a radiolabeled form of octreotide (the synthetic octapeptide derivative of the active
portion of somatostatin), is used to detect, localize, and evaluate such somatostatin-expressing tumors
by binding to these receptors (see Figure 44).
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I-123 ioflupane is a chemical derivative of cocaine. It binds to pre-synaptic dopamine transporters,
which are primarily located in the striatum (i.e., caudate nuclei and putamen). Loss of dopamine
transporter density, as occurs in Parkinson’s Disease, results in reduced uptake of the
radiopharmaceutical compared to normal (see Figure 45).
Figure 45. I-123
ioflupane image shows
normal uptake in the
caudate nucleus and
putamen, which appears
as a crescent (or comma)
shape, on both sides
(arrows).
Figure 44. In-111 pentetreotide image shows
increased uptake in a somatostatinFigure 43. Pre-therapy In-111
expressing carcinoid tumor in the left lung
ibritumomab tiuxetan image shows
(arrow). Also seen is normal uptake in liver
uptake in an abdominal lymphoma
and spleen, and normal excretion by the
(arrow). Also seen is normal retention
kidneys into the urine (bladder).
in blood pool, normal high uptake in
liver and spleen, and normal low
uptake in bone marrow and kidneys.
This patient was subsequently treated
with Y-90 ibritumomab tiuxetan. Note:
Prior to radiopharmaceutical
administration, the patient is pretreated with rituximab to saturate CD20
receptors on normal B cells.
A number of investigational radiopharmaceuticals currently in clinical trials are localized by receptor
binding. For example, several agents have been developed that bind to beta amyloid deposits in the
brain, which is the hallmark of Alzheimer’s Disease (see Table 2).
Table 2
Selected radiopharmaceuticals that bind to beta amyloid.
C-11 PiB (Pittsburgh compound B)
F-18 flutemetamole (fluoro-PiB)
F-18 florbetapir (AV-45)
F-18 florbetaben (BAY 94-9172)
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ALTERED BIODISTRIBUTION RELATED TO IMPROPER PREPARATION
Improper preparation or other problems that might occur during
preparation can result in abnormally high levels of radiochemical
impurities. Radiochemical impurities have different
biodistributions from the intended product, and abnormally high
levels of these impurities can potentially interfere with the
imaging procedure. Hence it is important for nuclear pharmacists
to recognize common radiochemical impurities and their
localizations.
The predominant radiochemical impurity in Tc-99m labeled
radiopharmaceuticals is Tc-99m pertechnetate. Free
pertechnetate is localized in salivary glands, thyroid, stomach, GI
tract, and urine/bladder (see Figure 46).
In some cases, small particles or precipitates may be formed.
These small particles are then phagocytosized by the liver (see
Figure 47).
Figure 46. Very
old, poor-quality
bone scan shows
Tc-99m
pertechnetate
impurity localization
in salivary glands,
thyroid, stomach, GI
tract, and
urine/bladder
Figure 47. Very
old, poor quality
bone scan shows
colloid impurities
taken up by the
liver (arrow).
In other cases, particulate impurities may be much larger in size
(e.g., flocculation). Particles larger than 10 microns will lodge in
the first capillary bed that they encounter (i.e., lungs following
intravenous injection) (see Figure 48).
In some situations there may be radiochemical impurities that are
excreted in the urine by glomerular filtration. Such impurities
are typically hydrophilic, ionized, non-protein bound, and have a
MW of <5000. One example of these impurities is the group of
Figure 48. Tc-99m sulfur colloid liver
scan shows uptake of large particle
impurities in the lungs (arrows).
radiopharmaceuticals that employ transfer ligands as intermediate chelating agents subsequent to
complexation with the final product (see Table 3). If the final complexation reaction is of substandard
yield, residual radiolabeled transfer ligand will be excreted in the urine.
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Table 3
Selected transfer ligands
Transfer ligand
Radiopharmaceutical
acetate
In-111 capromab pendetide,
In-111/Y-90 ibritumomab tiuxetan
citrate
Tc-99m sestamibi,
In-111 pentetreotide
edetate
Tc-99m bicisate (ECD)
gluconate
Tc-99m tetrofosmin
tartrate
Tc-99m mertiatide (MAG3)
In some other situations there may be radiochemical impurities that are excreted by the liver into the
biliary system. Such impurities are typically lipophilic, possess both polar and non-polar groups, and
have a MW of 300-1000. One example of this type of impurity is a radiochemical impurity in MAG3
that undergoes hepatobiliary excretion.
In yet other situations there may be radiochemical impurities that remain in the blood pool. These
situations generally involve impurities that label to red blood cells or are highly protein bound.
Improper preparation of In-111 labeled radiopharmaceuticals may result in several radiochemical
impurities. In+3 is soluble in acid, but precipitates at pH >3; these precipitate particles are
phagocytized by liver and spleen. Excess free In+3 in the blood binds avidly to plasma transferrin, and
hence exhibits prolonged blood pool retention. In some cases, there may be In-111 labeled peptides as
impurities, which are excreted by the kidneys into the urine. In other cases, there may be In-111
labeled large particles (e.g., antibody aggregates) which localize in the liver and/or embolize in the
lungs.
Improper preparation of I-123 or I-131 labeled radiopharmaceuticals may result in radiochemical
impurities. The predominant impurity of concern is free I-. Free iodide localizes in the thyroid, so
administration of radioiodinated radiopharmaceuticals typically require pre-treatment or concurrent
administration of SSKI to block thyroidal uptake.
Improper preparation of F-18 labeled radiopharmaceuticals may result in radiochemical impurities.
The predominant impurity of concern is free F-. Free fluoride localizes in the bone and is excreted in
the urine.
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INTERESTING CASE
Tc-99m sestamibi is widely used for parathyroid imaging. Tc-99m sestamibi is taken up in both
parathyroid adenomas and normal thyroid. However, it tends to be retained in parathyroid adenomas
whereas it slowly washes out of normal thyroid. Hence, delayed imaging is typically performed in
order to better identify parathyroid adenomas. Many physicians like to also perform a thyroid only
scan, using Tc-99m pertechnetate, as an adjunct in these procedures.5
A patient seen in our clinic underwent a parathyroid/thyroid imaging procedure using Tc-99m
sestamibi and Tc-99m pertechnetate (see Figure 49).
A technologist asked me: Why is
there a discrepancy in thyroid
uptake between the two scans?
The immediate, albeit only
partial, answer is that the two
radiopharmaceuticals have
Figure 49. (A) Tc-99m sestamibi parathyroid SPECT image shows
uptake in parathyroid/thyroid, salivary glands, heart, and liver. (B) Tc99m pertechnetate thyroid SPECT image performed 4.5 hours later
shows negligible uptake in the thyroid.
different mechanisms of uptake.
Tc-99m sestamibi localizes by
passive diffusion (with
intracellular retention) whereas Tc-99m pertechnetate is taken up by active transport via the Na+/Isymporter. A key piece of information in this particular case is that the patient was being treated with
levothyroxine. Levothyroxine inhibits pituitary release of TSH, so there was lack of TSH stimulation
of the thyroid to take up iodide (or pertechnetate). However, TSH has no effect on passive diffusion
uptake. This difference in uptake mechanism explains why Tc-99m pertechnetate was not taken up in
this patient’s thyroid whereas Tc-99m sestamibi had normal uptake in the same thyroid.
SUMMARY
A variety of mechanisms are involved in the localization of radiopharmaceuticals. It is important to
recognize that uptake often involves other processes as well (e.g., delivery to the tissue, retention in
cells). Radiopharmaceutical preparation problems can yield radiochemical impurities that demonstrate
altered biodistribution.
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REFERENCES
1. Kowalsky RJ, Falen SW. Radiopharmaceuticals in Nuclear Pharmacy and Nuclear Medicine,
Third Edition. Washington, DC: American Pharmacists Association. 2011.
2. Theobald T (ed), Sampson’s Textbook of Radiopharmacy, Fourth Edition. Gurnee, IL:
Pharmaceutical Press. 2011, pp 219-249.
3. Weatherman KD, Crisp W, Weber H. The physiological basis of radiopharmaceuticals. In: Smith
BT (ed): Nuclear Pharmacy. Gurnee, IL: Pharmaceutical Press. 2010, pp 55-66.
4. Vallabhajosula S, Killeen RP, Osborne JR. Altered biodistribution of radiopharmaceuticals: role of
radiochemical/pharmaceutical purity, physiological, and pharmacologic factors. Semin Nucl Med.
2010; 40:220-241.
5. Greenspan BS, et al. The SNM practice guideline for parathyroid scintigraphy 4.0. Society of
Nuclear Medicine. Revised 2011. Available at:
http://interactive.snm.org/docs/Parathyroid_Scintigraphy_V4_0_FINAL.pdf
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ASSESSMENT QUESTIONS
1.
All of the following are examples of common biologic compartments, EXCEPT:
a.
b.
c.
d.
2.
Use of which of the following radiopharmaceuticals is based on a classic example of uniform
dispersion within a compartment?
a.
b.
c.
d.
3.
cell sequestration
cellular migration
facilitated diffusion
leakage from compartment
Movement (flow) within a compartment is the mechanism of localization for each of the
following procedures, EXCEPT:
a.
b.
c.
d.
6.
pulmonary embolism.
acute cholecystitis.
COPD.
hydronephrosis.
Which of the following mechanisms is the basis for visualization of a site of GI bleeding?
a.
b.
c.
d.
5.
I-123 sodium iodide
I-125 RISA
I-131 tositumomab
Tc-99m exametazime
All of the following are examples of diseases/conditions that present non-uniformities within a
compartment, EXCEPT:
a.
b.
c.
d.
4.
lung airways.
lymphatic channels.
peritoneal cavity.
Meckel’s diverticulum.
gastric emptying.
lacrimal drainage.
ureteral reflux.
myocardial perfusion.
Regarding passive diffusion, which of the following is true?
a. In some biological systems, net movement can be from an area of low concentration to
an area of higher concentration.
b. Its rate is a function of the concentration gradient.
c. Input of external energy is needed for diffusion across biologic membranes.
d. It may be competitively inhibited by similar molecules.
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7.
Localization of Tc-99m tetrofosmin in the myocardial muscle is dependent on all of the
following, EXCEPT:
a.
b.
c.
d.
8.
Regarding glomerular filtration, which of the following situations would NOT be a problem?
a.
b.
c.
d.
9.
GLUT.
Na+/I- symporter.
Na+/K+ pump.
norepinephrine transporter.
Localization of Tc-99m pertechnetate in a Meckel’s diverticulum is by the mechanism of:
a.
b.
c.
d.
13.
It can be competitively inhibited by similar molecules.
It can transport molecules against a concentration gradient.
It requires the input of external energy.
Saturation is theoretically possible but unlikely in biological systems.
Examples of active transport systems include all of the following, EXCEPT:
a.
b.
c.
d.
12.
It can be saturated.
It requires a concentration gradient.
Its rate is solely a function of the concentration gradient.
It uses a carrier (membrane transporter).
Characteristics of active transport include all of the following, EXCEPT:
a.
b.
c.
d.
11.
cardiovascular shock (abnormally low blood pressure)
highly and tightly protein-bound
high concentration of similar molecules in the blood
MW > 5000
Characteristics of facilitated diffusion include all of the following, EXCEPT:
a.
b.
c.
d.
10.
glucose blood levels.
coronary blood flow (delivery to the tissue).
passive diffusion in the cells.
retention in the cells.
ion exchange.
secretion.
passive diffusion.
receptor binding.
“Colloid shift” refers to a shift in the biodistribution of Tc-99m sulfur colloid in patients with
diffuse liver disease, resulting in decreased uptake in liver with increased uptake in:
a.
b.
c.
d.
spleen and bone marrow.
kidneys/urine.
lungs.
lymphatic system/lymph nodes.
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14.
Which of the following is an example of cell sequestration?
a.
b.
c.
d.
15.
An appropriate particle size for capillary blockade is _____ microns.
a.
b.
c.
d.
16.
active transport.
chemisorption.
ion exchange.
receptor binding.
Which of the following is localized by cellular migration?
a.
b.
c.
d.
20.
active transport.
ion exchange.
chemisorption.
receptor binding.
Localization of Sm-153 lexidronam (EDTMP) at sites of bone metastases is by the mechanism
of:
a.
b.
c.
d.
19.
compartmentalization
passive diffusion
phagocytosis
capillary blockade
Localization of Sr-89 strontium chloride in skeletal hydroxyapatite is by the mechanism of:
a.
b.
c.
d.
18.
10-50
0.1-0.5
1-5
100-500
Treatment of liver tumors with Y-90 microspheres relies on which of the following
mechanisms of localization?
a.
b.
c.
d.
17.
localization of In-111 leukocytes in a site of infection
localization of Tc-99m RBCs in cardiac blood pool
localization of Tc-99m RBCs in a site of GI bleeding
localization of heat-damaged Tc-99m RBCs in the spleen
leukocytes
metastatic tumor cells
platelets
red blood cells
Which of the following antibodies does NOT bind to CD20 receptors?
a.
b.
c.
d.
ibritumomab
rituximab
tositumomab
capromab pendetide
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21.
Which of the following radiopharmaceuticals binds to somatostatin receptors?
a.
b.
c.
d.
22.
If faulty preparation of a Tc-99m labeled radiopharmaceutical results in a large fraction of free
pertechnetate impurity, localization would be expected in all of the following organs, EXCEPT:
a.
b.
c.
d.
23.
excreted in the bile.
phagocytosized by the liver.
excreted in the urine.
remain in the blood pool (bound to plasma proteins).
If the pH of an InCl3 solution is raised to 7.0, the impurity likely formed will be:
a.
b.
c.
d.
25.
salivary glands.
stomach.
liver.
thyroid.
For radiopharmaceuticals that are prepared using a transfer ligand, excessive radiolabeled
transfer ligand impurity will be:
a.
b.
c.
d.
24.
C-11 PiB
I-123 iobenguane
I-123 ioflupane
In-111 pentetreotide
excreted in the bile.
excreted in the urine.
phagocytized by liver and spleen.
remain in the blood pool (bound to plasma transferrin).
Regarding parathyroid/thyroid scintigraphy, which of the following statements is true?
a. Tc-99m pertechnetate and Tc-99m sestamibi are localized by the same mechanism.
b. Thyroid uptake of Tc-99m sestamibi is NOT affected by TSH.
c. Tc-99m sestamibi is localized in parathyroid adenomas by passive diffusion whereas it
is localized in thyroid by active transport.
d. Tc-99m sestamibi washes out of parathyroid adenomas whereas it tends to be retained
in normal thyroid tissue.
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